Cleaning is essential as a preliminary to many surface finishing operations. Cleaning is normally required, for example, prior to corrosion, preventive treatments and prior to the application of organic finishes and printing inks to the metal surfaces. Cleaning is especially important in the case of metal surfaces to which organic materials have been applied as an aid to rolling or forming since these materials must be removed in order to obtain a surface which is receptive to printing inks and organic finishes.
Cleaners have been utilized in the manufacture of aluminum and tin plate, drawn and ironed cans. In the manufacture of such cans, circular blanks of aluminum or tin-plated steel are first cupped and then passed through several drawing dies to iron the cup in order to form a unitary sidewall and can bottom structure. These forming operations are assisted, and the dies and metal surface protected by the application of lubricants to the aluminum or tin surface prior to or during the forming operation. Since it is desired to have a clean surface prior to any processing steps such as conversion coating and sanitary lacquer deposition in order to assure adhesion of subsequently applied organic coatings, the cleaning step after forming is critical to a successful manufacturing process.
Aluminum and tin plate cleaning or washer processes utilize and require the disposal of large quantities of water laden with chemical wastes. Because of environmental concerns, the water must be treated to reduce or remove chemicals from the waste stream, and new environmental regulations are requiring industries to spend a great deal of money on new waste treatment facilities.
During the cleaning process, organic soils such as forming and rolling oils, and inorganic soils such as metal fines, natural oxides and corrosion are removed from the metal surfaces. For example, if the metal is aluminum, the cleaning process removes aluminum fines, aluminum oxides and water-soluble aluminum derivatives. When the metal is tin plate, the inorganic soils removed during the cleaning process include tin fines, fin oxide and water-soluble tin derivatives.
The inorganic soils which are present on the aluminum or tin cans are digested by the cleaner and remain in solution as a soluble metal salt. The oils which are utilized in preparing the aluminum or tin-plate coils and cans including forming oils and rolling oils which remain on the can surface are removed by alkaline or acid cleaners, and such organic soils are either insoluble or emulsified and dispersed throughout the cleaning solution. The organic soil must be removed from the surfaces of the cans because these soils are responsible for off flavors in the industry known as Labox. Additionally, incomplete removal of organic soils causes poor lacquer adhesion staining, spotting, or imperfections in subsequent coating and printing operations.
As the container cleaning cycle continues, the inorganic and organic soils accumulate in the cleaning solutions and in subsequent rinse solutions as contaminates. If uncontrolled, these contaminates can render the cleaning solutions and rinse solutions ineffective. The reduction of the contaminates in the cleaning and rinse solutions previously has been achieved through overflowing and/or discarding of the used solutions as fresh, unused solution is added. As noted, these methods of reducing contaminates result in the disposal of large quantities of water laden with chemical wastes. Moreover, this process requires that the cleaning solutions be replaced by fresh water and additional chemicals thereby increasing costs.
U.S. Pat. No. 4,094,701 (Fekete) describes a method for cleaning a tin surface. In particular, Fekete describes a cleaner which is an alkaline aqueous composition containing one or more surfactants and an organic tannin component to inhibit etching of the tin surfaces. The aqueous alkaline compositions are also reported to be useful for cleaning surfaces of other substrates such as steel and aluminum.
U.S. Pat. No. 4,599,116 (King et al) describes aqueous alkaline cleaning compositions for aluminum containers which are effective in removing aluminum fines and organic soils from the aluminum container surfaces. The aqueous alkaline cleaning compositions contain an alkalinity agent, a complexing agent, one or more surfactants and, optionally, foam depressing agents. The patentees also mention the use of a prewash to remove a portion of the aluminum fines and soluble oil prior to the alkaline cleaning stage. The complexing agent is included in the alkaline bath in an amount effective to complex at least some of the metal ions in the operating bath which would otherwise tend to form bath-insoluble precipitates. Examples of the complexing agents include sugar acids and salts such as sodium gluconate and sodium citrate. The surfactants which are included in the alkaline cleaning composition are selected to remove the organic soils present on the substrate being cleaned and to prevent a build-up of such organic soils in the cleaning solution.
Acid cleaners also have been used to clean aluminum surfaces and to remove aluminum fines deposited on the interior and exterior walls of the aluminum containers. U.S. Pat. No. 31,198 (Binns) describes an acidic cleaner for removing and dissolving aluminum fines and lubricating oils from an aluminum surface. The cleaner comprises hydrofluoric acid, sulfuric acid, one or more surfactants and water.
U.S. Pat. No. 4,863,612 (Kirman et al) describes a method and apparatus for recovering ions from an aqueous solution and producing deionized water for reuse. The method and apparatus are particularly useful in a nickel-plating process and involves passing the aqueous solution through a first cation removing assembly which contains a cation exchange present having a high affinity for nickel ions. The water stream leaving the first cation assembly passes through a second cation assembly and then to an anion removing assembly so that deionized water is produced. The first cation assembly is regenerated producing a regeneration solution containing a high concentration of nickel, and the regenerated solution can then be returned to the nickel plating bath. The ions recovered in the second cation removing assembly and the anion removing assembly are recovered and discarded.
U.S. Pat. No. 5,002,645 (Eastland et al) describes a method for separating and recovering metal values from a waste stream containing metal hydroxides, particularly chromium hydroxide. The waste stream is subjected to an oxidation process, pH adjustment, a chelating ion exchange resin and ultrasound. The process is used for separating and recovering metals from electroplating or electroless processes where the metals may be chromium, iron, cobalt, zinc, cadmium, nickel, copper, silver or aluminum.